Carbon for capture: Sulphonated graphene sheets extract PAHs from river water by micro-SPE

Ezine

Published: Apr 9, 2012

Author: Steve Down

Channels: Sample Preparation

Carbon sorbents

The new generation of carbon-based materials has found application in many branches of science, including separation science. Carbon nanotubes and fullerenes, and some functionalised forms with added chemical groups, have found a growing number of uses in the extraction of compounds like drugs and pollutants. A further carbon form is graphene sheets, which are also generating a buzz.

Graphene is the two-dimensional form of carbon, existing as monolayers of carbon atoms in a honeycomb structure. It has a high specific surface area which has already been exploited in several applications such as biosensors and a storage medium for hydrogen.

More recently, graphene has been used for the extraction of several groups of compounds by solid-phase extraction (SPE) and solid-phase microextraction (SPME), where it was used to coat the extraction fibre. Now, scientists in Singapore have employed modified graphene sheets for the first time as a sorbent for micro-SPE.

Hian Kee Lee, Hong Zhang and Wei Ping Low from the National University of Singapore did not use normal graphene but a functionalised form containing p-phenyl-sulphonic acid groups. The modification imparts good water solubility to the material and the consequent electrostatic interactions ensure that the sheets do not aggregate in solution, leaving the surfaces free for contact with the target compounds.

Graphene tea bags

The functionalised sheets were prepared by a published method which uses chemical exfoliation of graphene oxide in a process involving pre-reduction, sulphonation and reduction. A small number of p-phenyl-SO3H groups were introduced into graphene oxide before its reduction to graphene and the structures were confirmed by FTIR spectroscopy and transmission electron microscopy. Elemental analysis revealed S:C ratios of 1:23 and 1:28 in two types of material produced.

The graphene sheets were produced in powder form which was used directly as the sorbent for a micro-SPE device. A small portion was sealed into a membrane envelope of polypropylene, like tea in a tea bag, and cleaned by ultrasonication in turn with dichloromethane and water.

The porous membrane protected the graphene sorbent so that no clean up of real samples was required. Even river water could be analysed directly without a pre-filtration step. The micro-SPE device was simply placed in the solution for a pre-determined time, then removed and extracted with toluene for GC/MS analysis in selected ion monitoring mode.

Lee tested and optimised the device for the extraction of seven polycyclic aromatic hydrocarbons (PAHs) from water using stock solutions, before applying the method to real samples of water collected from the Singapore River.

Good performance for PAHs

Both types of functionalised graphene sheet were more absorbent than conventional C8 and C18 sorbents. The sheets with the higher degree of sulphonation performed the better of the two, so were used for all subsequent experiments.

The extraction yield peaked after 30 minutes, before dipping over the next 10 minutes and rising again by 60 minutes. The dip was tentatively attributed to back-extraction, which was greater for the lower-molecular-weight PAHs.

Toluene was a more efficient extractant than methanol, acetonitrile and hexane and the optimum desorption time was just 10 minutes. Under these conditions with the subsequent GC/MS analysis, the detection and quantitation limits were in the ranges 0.8-3.9 and 2.7-12.9 ng/L, respectively.

These compared favourably with published figures for PAHs extracted by solid-phase nano-extraction with gold nanoparticles followed by HPLC-fluorescence, SPE with multiwalled carbon nanotubes (HPLC-UV or GC/MS) and SPE with C30 (GC/MS).

Next, the river water was extracted using the micro-SPE device and analysed by GC/MS. All seven PAHs were detected, with naphthalene measured at 0.11 µ/L and the other PAHs (acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene and pyrene) found in low amounts but not measured. Their presence was confirmed by spiking the river water with PAH standards, which also led to estimations of the relative recoveries of 81.6-113.5%.

This preliminary study confirms the utility of sulphonated graphene sheets fabricated in a micro-SPE device for the extraction of PAHs from water. The researchers declared that they should also be suitable for other hydrophobic benzenoid-type compounds.

They concluded "To increase the extraction performance of graphene, structural and compositional modifications are required to retain its intrinsic high surface area. Further development relating to this is being conducted in our laboratory."

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